This invention relates to a method of incubating alevins. It enables the alevin or sac fry density to be considerably greater than has been feasible heretofore without adversely affecting the sac fry or alevins.
The invention has particular relation to the incubation of salmonids, especially salmon and trout, but also applies to other fish which do not have a larval stage. The artificial spawning of salmon and trout employs incubators wherein the fertilized egg develops, first, to the "eyed egg" stage, next, to the hatching stage, and finally to absorption of the yolk sac by the alevin or sac fry, prior to swimup. For example, the total incubation period of coho salmon eggs is approximately 68 days at about 11.degree. C., of which the third stage occupies approximately 21 days, from the hatching of the egg to the complete absorption of the yolk sac. At the end of the third stage the alevins or sac fry have become fry and are transferred to another container in the hatchery.
The development of a fertilized egg from the time of its fertilization through the end of the alevin stage (i.e., "swimup") is dependent upon many factors. Significant factors include the water flow rate, the oxygen content of the water, the temperature of the water, the pH of the water, its alkalinity, its ammonia-nitrogen content, its turbidity, its carbon dioxide content, and its carrying of disease-inducing organisms. Some of these are relatively easily adjusted and the ranges are well known. The temperature of the water varies with the type of fish; for salmon and trout it is typically in the range of 7.degree.-17.degree. C. (45.degree.-63.degree. F.) with optimum conditions for salmon and trout being around 11.degree. C. (52.degree.-53.degree. F.). Interfering suspended solids, presence of disease organisms, carbon dioxide, pH and alkalinity are also known factors that must be considered and adjusted.
During the entire incubation period water flow is essential, and it is especially important during the third or alevin stage, for two main reasons: (1) to remove waste and (2) to carry oxygen to the alevins. To amplify, the proper flow of water during the alevin stage is important in eliminating nitrogenous wastes, which can be measured as ammonia toxicity, and in maintaining the oxygen requirement, which increases during the alevin stage. However, the water flow rate can be too high during the period of incubation, and if it is, it weakens the alevin, reducing their ability to gain weight and increasing their susceptibility to various disease conditions.
Probably the second most important factor, next only to the flow rate of the water, is the oxygen content of that water. Oxygen content, I have found, is extremely important in the capacity of any hatching apparatus. In the past, I have found that hatching trays or other containers have sometimes been overloaded with alevins with respect to the oxygen content. When the oxygen concentration fell too low the rate of fish mortality has increased to unacceptable levels. I have also found that oxygen levels high enough to maintain the fish may still be too low for proper growth.
Hatchery equipment has become quite expensive; so that tray space is at a premium. As a result, it is uneconomical to utilize the hatchery trays at amounts below capacity. Yet these lower densities have been employed heretofore because otherwise the loss in fish has been too great. I find that this loss in fish has been caused by the alevins receiving insufficient oxygen under crowded conditions.
In the past the oxygen content of the water used in hatcheries has varied rather widely. The water may be naturally saturated with air, or saturation may be achieved by any of several air-impinging techniques, such as bubbling air through a water column, using airlift pumps, impinging sprays of water on open containers of water, or trickling water through beds of rocks, oyster shell, or other media. All of these techniques yield water that is in equilibrium with air. All of these conventional means for supplying dissolved oxygen to the alevins have had a serious disadvantage: since air contains abour four times as much nitrogen as it does oxygen, the partial pressure of oxygen is inadequate for the purpose. Table I shows the solubility of oxygen in water in equilibrium with air.
TABLE I ______________________________________ Solubility Of Oxygen In Water Exposed To Water-Saturated Air Temperature Dissolved in .degree. C. Oxygen (mg/l ______________________________________ 0 14.6 4 13.1 8 11.9 12 10.8 16 10.0 20 9.2 24 8.5 28 7.9 32 7.4 36 7.0 40 6.6 ______________________________________
From Table I it will be seen that when water is at about 12.degree. C. or approximately 54.degree. F., the dissolved oxygen in the air-saturated water is approximately 10.8 mg/l. This, in itself, exceeds what the alevins need. However, as the water flows through the incubator, usually going through a series of horizontally disposed trays located vertically one over the other, the oxygen content of the water entering the top tray and leaving from the bottom tray is inevitably diminished. Most of it is consumed by the alevins. As a result, air-saturated water when introduced at non-stressing flow rates is not able to support such a series of densely packed trays of alevins. For example, one commercial incubator employs sixteen trays in each vertical column, with the water passing successively from the top tray through each of the sixteen trays, leaving from the bottom tray. As a result, the water leaves the last tray at a perilously low level of oxygen unless the alevins are relatively sparsely distributed.
One of the objects of the present invention is that of providing a method by which alevins can be reared in such incubators under crowded conditions utilizing substantially the capacity of the trays or other such containers.